1. Field of the Invention
The present invention relates to a wet scrubber process for removing Total Reduced Sulfur compounds (TRS), especially H.sub.2 S and mercaptans, from industrial gases. More particularly, the present invention relates to a wet scrubber absorption process employing an aqueous absorbent slurry of particulate manganese dioxide as an oxidative absorbent.
2. Description of the Prior Art
Various processes have been proposed for reducing the content of sulfur compounds in industrial waste gases. Most of these processes relate to the removal of sulfur oxides. Because of their substantial acidic character, these sulfur oxides are readily absorbed by alkaline absorbents. Unfortunately, the same absorbents are not generally satisfactory for removing a number of important industrial by product sulfur compounds such as hydrogen sulfide, mercaptans and other organic sulfur containing compounds, since the acidic character of these compounds is generally much less pronounced. This is particularly true of the mercaptans for which there is no effective removal process being used commercially.
In British Patent Specification No. 1,576,534 a process for removing hydrogen sulfide from a hot reducing gas is disclosed. The process uses an absorbent comprising a mixture of finely divided manganese oxide, i.e., manganese of oxidation state 2, and finely divided aluminum oxide. Both the absorption and regeneration steps of the patented process require gas temperatures substantial higher than those typically encountered in industrial applications, for example, in pulping mills. Furthermore, a hot sulfur oxide-containing gas is produced by the disclosed high temperature regeneration technique, creating additional disposal problems in many cases. Moreover, it does not disclose that the absorbent can remove mercaptans from gas streams.
U.S. Pat. No. 3,898,320 to Atsukawa uses a dry, powdered absorbent comprising a hydrated manganese oxide to remove sulfur oxides from gas streams. As described, the sulfur oxides in the gas react with the hydrated manganese oxide absorbent to produce manganese sulfate. The manganese sulfate is subsequently solubilized in water, converted to manganese hydroxide in the presence of ammonium hydroxide and is thereafter reconverted to the hydrated absorbent by oxidation with an oxygen-containing gas. The oxidation is conducted by bubbling the oxygen-containing gas through an aqueous dispersion of the manganese hydroxide. As in the prior process, there is no indication that this absorbent can be used to remove H.sub.2 S or reduced organic sulfur compounds, particularly mercaptans.
U.S. Pat. No. 3,492,983 to Lowicki, et al., describes a process for removing sulfur containing compounds including H.sub.2 S and organic sulfur compounds which employs a complex multicomponent absorbent. This absorbent includes a metal oxide, hydrated oxide or hydroxide or preferably mixtures thereof, for example, manganese dioxide and magnesium oxide, in combination with an alkali metal or alkaline earth metal oxide or hydroxide, for example, sodium hydroxide. The absorption process is conducted at a relatively low temperature but an oxidic roasting at an elevated temperature above at least about 750.degree. C. is required to regenerate the absorbent. As in the prior British patent, problems with disposing a high temperature regeneration gas containing sulfur dioxide are created in many cases.
Iannicelli et al, U.S. patent application Ser. Nos. 486,672 and 530,267, now U.S. Pat. Nos. 4,552,735 and 4,552,734 filed on Apr. 20, 1983 and Sept. 8, 1983, respectively, describe processes which have proved to be successful for removing TRS compounds from industrial waste gases. In these processes, a waste gas is contacted with an active manganese dioxide absorbent to remove TRS contaminants from the gas and produce a substantially dry spent absorbent powder. Spent absorbent can be regenerated, for example, by forming an aqueous slurry and then contacting it with an oxygen-containing gas.
A particularly troublesome source of TRS pollutant emissions are paper mills employing the kraft pulping process. The economics of paper making require that chemicals used to digest raw wood be recovered to the fullest extent possible. Thus, a major part of any paper mill is directed to the recovery and regeneration of such chemicals. Unfortunately, the various steps involved in the recovery and regeneration operation unavoidably generate waste gases which contain entrained particulates and odorous contaminants, principally TRS compounds. One troublesome pollutant source, for example, is the lime kiln where calcium oxide used to convert green liquor to white liquor is regenerated.
In many paper mills a wet scrubber is used to collect entrained particulates from the flue gas emissions, which in the case of a lime kiln is primarily lime dust. The dust-laden gases pass through a narrow duct where a water spray contacts the dust particles and removes them from the gas stream. In the case of the lime kiln emission, the recovered lime dust eventually is recycled for reuse. Unfortunately, the TRS contaminants usually contained in the various flue gas emissions from a paper mill are substantially unaffected by the water spray and thus are discharged with the flue gas into the environment.
Moreover, even if the scrub solution is made very highly alkaline, e.g., having a pH in the range of about 11 to 13, TRS pollutant emissions are only minimally abated. While hydrogen sulfide would be at least partially removed from the gas, other TRS compounds including mercaptans would be substantially unaffected. This approach, however, is not really practical. Besides the added expense required for safely handling such a scrubber liquid, its high alkaline content would unavoidably upset the chemistry in the pulping operation and thus would necessitate subatantial additional modification in the overall kraft mill recovery and regeneration operation.
Because such paper mills already have a wet scrubber in place, there is a strong reluctance to add any additional gas scrubbing equipment such as the spray drying absorption equipment of Ser. No. 486,672 now U.S. Pat. No. 4,552,735 or the fluid bed absorption equipment of Ser. No. 530,267, now U.S. Pat. No. 4,552,734 to the gas cleaning train to remove TRS contaminants. Thus, a need exists in the art for a wet scrubbing process having the ability to remove TRS contaminants from a gas stream.
It is an object of the present invention to provide a process for efficiently removing H.sub.2 S and organic sulfur-containing compounds, and particularly mercaptans, from industrial gas streams, such as the various flue gas emissions from a paper mill.
It is another object of this invention to provide a process for removing the aforementioned sulfur compounds from industrial gas streams which permits a smooth and continuous operation suitable for automation.
It is a further object of this invention to provide a process for removing these sulfur compounds from industrial gas streams which employs a readily available absorbent that can be simply and efficiently regenerated by ambient temperature oxidation procedures.
It is yet another object of the present invention to provide a process for removing these sulfur compounds from industrial gas streams which is completely compatable with the wet scrubber processes widely used for removing solid pollutants from such industrial gases.
It is still a further object of the present invention to provide a process for removing sulfur compounds from industrial gases by a wet scrubber process which provides for separate recycle of recovered entrained particulates and active scrubber liquid.